1. Field
The present disclosure relates to an induction heating fusing device and an image forming apparatus.
2. Description of the Related Art
An image forming apparatus is provided with a fusing device for fusing a transferred toner image on a recoding medium, such as a sheet. The fusing device includes a fusing roller or a fusing belt (heating roller) thermally fusing a toner transferred on the sheet, and a pressurizing roller pressure-welded to the fusing roller or the fusing belt to pressurize the sheet.
An induction heating fusing device which is provided inside or outside the fusing roller or the fusing belt with an induction heating coil to heat the fusing roller or the fusing belt is widely employed. An induction heating method heats the fusing roller or the fusing belt by allowing a magnetic flux generated by the induction heating coil to flow through a conductor part of the fusing roller or the fusing belt to allow an eddy current to flow through the inside of the fusing belt or the fusing roller and to heat the fusing roller or the fusing belt with Joule heat generated by this eddy current.
Power control methods in a related art induction heating fusing device are classified into a method of controlling a driving frequency with an LCR resonance circuit, and a method of controlling a current amount by performing a PWM control while a resonance circuit is resonated at a resonance frequency f. Related art methods of changing an output power by controlling a driving frequency are disclosed in Japanese Patent Publication Nos. 2008-51951 and 2008-145990.
In a related art induction heating fusing device 900 designed to convert a current amount by performing a PWM control in the state of a resonance frequency f to control a current amount, a construction of an inverter power supply is shown in FIG. 1. A current from an AC power supply 901 is full-wave rectified using diode bridge 904, passes through a noise filter 905, and is supplied to a half bridge output circuit 906. In FIG. 1, reference numerals 902 and 903 indicate a fuse, and a surge voltage protecting varister, respectively.
The half bridge output circuit 906 is a switching element, and includes, for example, an insulated gate bipolar transistor (IGBT), a field effect transistor (FET), etc.
In the construction of FIG. 1, the half bridge output circuit 906 employs IGBTs 907 and 908 as switching elements. An LC serial resonance circuit includes a induction heating low loss coil 912, and condensers 913, 914, and generates a magnetic field while a high frequency current flows through the induction heating low loss coil 912 being composed of a Ritz wire (an electric wire comprised of thin stranded copper wires. The magnetic field generated by the induction heating low loss coil 912 is concentrated on the fusing roller or the fusing belt 910 made of a high permittivity material to allow an eddy current to flow through a surface of a heat radiator, so that the fusing roller or the fusing belt itself generates heat.
A phase comparison between a driving voltage of an output of a current transformer 909 for detecting current and phase difference of the induction heating low loss coil 912 and a driving voltage (one side) of a half bridge output by IGBTs 907 and 908 is performed by a phase comparator 928 (e.g., commonly used PLL IC (74HC4046, etc.)) in a phase-locked loop (PLL) circuit 927, and a phase comparison result of the phase comparator 928, which also receives a current outputted from a limiter circuit 931, is outputted to an RC saw oscillation type voltage control oscillator (VCO) 929. An oscillation frequency of the VCO 929 is feedback-controlled such that the phase difference between the driving voltage of the output of the current transformer 909 and the driving voltage of the output of the half bridge disappears. A resistance 926 is used for allowing current to flow through the resistance 926 from the current transformer 909.
In a PWM controller 919, a PWM On duty value calculated through a proportional, integral, differential (PID) operation by a PID controller 917 at a CPU 915 from information of a heat radiator temperature sensor 911, and an output of the current transformer 909 which has been rectified by a rectifying circuit 930 are amplified by an error amp 920, the amplified value and an output of VCO 929 are compared by a comparator 921, and a comparison result is outputted to a PWM driver 922, and the PWM driver 922 may output a PWM signal to photodiodes and phototransistors 923 and 924. The CPU 915 further includes an AD converter (ADC) 916 and a DA converter (DAC) 918.
In the power control methods of the related art induction heating fusing device that controls a driving frequency by using an LCR resonance circuit, in case a resonance frequency of the resonance circuit is changed, it may be impossible to control the induction heating fusing device, and for cope with such a circumstance, like the invention disclosed in Japanese Patent Publication No. 2008-51951, there is a need to obtain a frequency which allows power to be peaked and to control the obtained frequency as a lower limit frequency. Also, in controlling a small power, the frequency is so high that a switching loss of the half bridge output element may be increased and thus efficiency may be reduced. As a solution, there is a need to divide the power control method into a large power control method, a middle power control method, and a small power control method. Also, when the half bridge element is switched in a state that a driving frequency deviates from the resonance frequency, a zero voltage switching is not performed, so that a device loss may be generated, and degeneration or heat fracture due to heat generation may be caused.
Meanwhile, in the methods that change the current amount by performing a PWM control in a state that a resonance circuit is resonated at a frequency of f to control the current amount, since a phase comparator, a voltage control generator and a PWM controller are configured by an analog circuit, there is a need to consider a deviation in component constant or variation in temperature, or to change component constant according to the specification, like setting of a resonance frequency tracking range. Also, in case there is a frequency region (e.g., a specific RF or a resonance frequency of a fusing device, such as a fusing belt) that may not be used for a specific purpose, it is difficult to deviate from such a frequency range and automatically track the resonance frequency.
Further, by performing only the PWM control, a very small current region may not be controlled. This is because the switching speed of a switching element, for example, an IGBT is not fast to such a degree that may control a very small current by using a PWM.